In general, most new tires are made by wrapping a layer of uncured tread rubber around an uncured tire casting and placing the combination in a tire mold where it is subjected to increased temperature and pressure. The result is a "cured" tire in which both the tread and casing are cured simultaneously. This process is effective in making tires because the tread and casing are cured together and only once. However, when these conditions are applied to the retreading of an already cured casing the process does not work nearly as well. The reason for this lies in the properties of once-cured rubber; for while the new tread is being "cured" for the first time, the previously-cured casing is being subjected to a re-curing operation. The result is an over-cured casing whose fibers are severly weakened.
To avoid this problem a second method of recapping tires was developed. According to the method a precured tire tread is wrapped around a precured casing and both elements are bonded together by a "cold" vulcanization process.
In a preferred aspect of this method, an uncured cushion stock of rubber is applied to the casing to secure the tread strip and this combination is encased in an elastic and fluid impervious envelope. The encased tire is then mounted on a rim, inflated to its normal dimensions and placed in a pressure chamber where bonding is effected at temperatures between 212 and 312 degrees Farenheit.
Generally, an outlet valve is provided in the envelope so that entrapped air and moisture from between the tread strip and tire casing can escape to the atmosphere and thus assure a tight cure.
The advantages of using an envelope of this type are several.
First, there is little or no danger of deforming the tire, as in the case when a rigid sealing means is used. And secondly, the elastic envelope will assure retention of the tread material to the tire body during the vulcanization process.
Unfortunately, however, elastic retreading envelopes have a limited life. Although they can be reused so long as they remain intact; nevertheless, and in practice, it commonly occurs that the pulling on and securing of the envelope over the tire carcass, and its subsequent removal, weaken the envelope severely and it develops leaks and eventually fails. Severe wear is also caused as a result of the envelope being drawn down in between the tread lugs. Thus, in retreading processes, the integrity of the envelope is constantly being tested and once it is punctured its effectiveness is destroyed because it can no longer maintain the vacuum which is needed to maintain an effective seal.
In said parent application there is described an improved retreading envelope of such durability that it can be reused repeatedly so as to achieve economies exceeding those of prior envelopes. The retreading envelope of said application comprises a flexible, seamless and fluid impervious envelope of U-shaped cross-section. Also, by utilizing the extruding device described in said application, an envelope can be provided with predetermined thickness along any given portion of its U-shaped cross-section.